My previous U.S. Pat. No. 5,374,172 (hereinafter xe2x80x9cthe ""172 inventionxe2x80x9d ), entitled ROTARY UNIVANE GAS COMPRESSOR and issued Dec. 20, 1994 (and corresponding non-domestic patents), teaches a fluid-handling device that employs a single vane (hereinafter sometimes referred to as xe2x80x9cUniVanexe2x80x9d) which, in combination with its attending components, can pump, compress or expand fluids. Importantly, this single vane is tethered opposite its tip by two anti-friction bearings, one placed on each side of the vane. This unique arrangement precisely controls the radial location of the vane tip such that it operates within very close sealing proximityxe2x80x94but not in physical contact withxe2x80x94the internal surface of the stator cylinder.
This important and distinguishing feature of the UniVane compressor, by eliminating vane tip friction but effectively preserving the sealing of the dynamic interface between the vane tip and its attending stator wall, results not only in a very reliable machine but one of great energy efficiency due to the minimization of mechanical friction.
Another advantage of the ""172 invention is that it can be operated in an oil-less mode because the machine can be fitted with lifetime-lubricated sealed anti-friction bearings that, further, are not even within the flow field of the fluid being processed. At ordinary rotor shaft speeds, the centrifugal force tugging at the vane tip tether pin resulting from the rotating mass of the vane is modest.
However, being a function of the square of the rotor RPM, this centrifugal tether force quickly becomes excessive with increasing speed, thus rapidly setting a practical speed limit (RPM) for the rotor shaft of the ""172 invention. The present invention greatly decreases this limitation thus allowing significantly higher speed single vane or UniVane operation. Among other advantages, this greatly decreases the size and weight of the machine while simultaneously significantly increasing its throughput.
While this improvement is not of particular commercial importance to some oil-less applications, a new and challenging requirement has arisen. This application requires the efficient supply of large quantities of relatively low-pressure clean air over a very wide range of operation, i.e., energy demands of fuel cells for automobiles, trucks, buses and the like (hereinafter xe2x80x9cautomotive fuel cellsxe2x80x9d). In this application, of course, the size and weight of the air supply equipment is of great significance. Although achieved in a far more efficient and ecological manner, air-breathing fuel cells, like combustion engines, combine hydrogen and oxygen in order to produce power.
This new air delivery requirement for fuel cells has not been served well by conventional fluid-handling devices because they were neither conceived nor designed for the unique air flow needs of fuel cells which, again, require relatively large amounts of flow at relatively low pressures. The uniqueness resides in the limitations of the only two fundamental types of mechanisms than can be used to compress, expand, and pump fluids: positive-displacement or momentum-conversion devices.
Basic Compressor Types
There are two fundamental means to provide compression (and pumping and expansion) of fluids: positive displacement machines and momentum-conversion machines. These types of devices are fundamentally different and their operating characteristics dictate whether or not they are adaptable to a given application. Positive-displacement machines achieve the compression of a gas by diminishing its volume through the relative motion of physical surfaces containing the gas. Prominent examples of such mechanisms include piston-cylinders and conjugate screws and scrolls.
Momentum-conversion devices, on the other hand, achieve compression by causing the gas to increase its speed, thereby absorbing kinetic energy, and then quickly slowing it down. This reduction in velocity converts the fluid""s kinetic energy to potential energy, thus compressing the gas. Such machines are known variously as centrifugal pumps, fans, and turbines, and all operate on the same physical principle.
The functional differences between positive displacement and turbine-type devices are manifested in quite dissimilar operating characteristics. Specifically, the flow rate of positive-displacement pumps is almost directly proportional to shaft speed and their pressure ratio is nearly independent of speed. Conversely, turbo-machines, which rely upon kinetic energy to compress gases, are very non-linear devices. Their flow rate is proportional to the cube of their speed and their pressure ratio varies as the square ofrotor RPM. On the other hand, turbo devices can operate at very high speeds and are, therefore, much smaller than conventional positive displacement machines for the same rate of flow delivery. These elemental distinctions turn out to be very important, depending upon the air delivery and operational requirements of the machine.
In the case of propulsion fuel cells, these differences are of fundamental importance because the power requirement for an automotive fuel cell can vary greatly from instant to instant. Also, it is advantageous to operate automotive fuel cells at a constant air pressure across a very large range of loads. This load range, known also as the xe2x80x9cturn-down ratio,xe2x80x9d is very significant for a land vehicle.
Interestingly, this principle is the root reason that gas turbines, used as a land vehicle prime mover, have proven unable to commercially compete with conventional internal combustion engines. Internal combustion engines, diesel or spark ignition, are positive displacement devices whose power and torque characteristics can far more easily accommodate the variable-load performance required by land vehicles than turbo-machines. It is therefore not altogether surprising that turbo compressors/expanders will prove to possess inadequate fundamental properties to enable it to adequately service automotive fuel cells. Conversely, the power demand of aircraft and large sea-going vessels, which is generally a single load, provides an excellent platform to use gas turbine propulsion.
The foregoing has meant to illustrate that while positive-displacement compressors possess the flow and pressure-ratio characteristics required for land vehicle fuel cell propulsion, they are much bigger than turbo-machines that have nonlinear characteristics difficult to deal with in this application. What is needed, therefore, is a positive displacement mechanism that can rival the physical size of turbo-machines. Such a device would therefore incorporate the RPM characteristics required of large xe2x80x98turn-downxe2x80x99 ratio fuel cells but small in weight and size for mobile applications. That is what the present invention achieves.
Although collateral factors are of importance, a preferred embodiment of the present invention employs the development of centrifugal forces (due to rotation) that are used to its advantage by insuring that the vane is designed and controlled so the center of gravity thereof always rotates (orbits) within the stator bore around the smallest radius of gyration consistent with the geometric limitations of rotor/stator off-set. This is achieved, for instance, by choosing the center of gravity of the vane such that when vane is at the 6 O""clock position shown in FIG. 3a, the vane center of gravity is in register with the center of the stator bore. While other points can be chosen with varying result, the stator center turns out to provide the smallest radius of cg gyration. The combination of configuration and elements provided by my invention leaves the tether guide pins to insure the precise location of the vane against only the mild inertial loads and ordinary pressure and frictional forces.
Another important feature inherent in this invention is the radial extension xe2x80x98tonguexe2x80x99 of the vane. This extension not only enables the positioning of the vane cg as desired, but also greatly enhances the load distribution of the vane against the drive side of the rotor slot by significantly increasing the amount of vane xe2x80x9ctucked inxe2x80x9d to the rotor slot as compared to the vane surface extending into the fluid being compressed.